The present invention relates to improvements in sensors for use in physical security alarm system circuits, and particularly to a magnetically retainable connector cable incorporating a magnetically operated switch for incorporation in an alarm system for detection of unauthorized removal of an object from its proper location.
It has long been known to use magnetically operated proximity switches in security systems to detect unauthorized opening of doors, windows, gates, and the like. Typically, a magnetically operated switch such as a magnetic reed switch is fixedly mounted on a stationary structure and a permanent magnet is appropriately mounted on the movable window, door, or gate. When the movable object is in a predetermined position, the magnet actuates the switch, appropriately opening or closing an electrical circuit connected to an alarm system control device designed to respond appropriate. The usefulness of such previously known magnetically operated switch assemblies has depended upon the ability to mount an actuating magnet in a position where it will reliably actuate a magnetically operated switch whenever the door, gate, etc., is in an appropriate position. Such a system depends, however, on the door, etc., returning predictably to a position which must usually be within about an inch of a reference location.
In many instances, it is desirable to have an electrical indication available to an alarm system control device whenever a valuable movable object is removed from its authorized location. For example, freight containers, trailer trucks, construction equipment, boats, and the like are frequently left unattended in open lots or alongside unattended piers where there is no need for them to be precisely located. It would be possible to provide connector cables incorporating special plugs, with corresponding jacks or sockets being provided on the movable articles to be protected, to complete an electrical circuit to an alarm system. Such plugs and sockets would be susceptible to short-circuits or corrosion of their contact points caused by exposure to the weather, however, and they would therefore present a serious problem of poor reliability.
Encapsulated magnetic reed switches have been used in intrusion alarm system sensor circuits in order to avoid exposure of switch contacts or electrical connector contacts to the weather, but as previously explained, the previous use of such magnetically operated switch assemblies has required an actuating magnet carried on a movable object to be relatively precisely located with respect to the magnetically operated switch. Previously known magnetically operated switch devices are therefore not readily useable to protect objects which are floating on water. Nor are all such devices well adapted for use on overhead doors, whose tracks often permit up to an inch or more of movement, on fence gates which often move in the wind, or to protect heavy equipment which must be left on a construction site between work shifts, in unpredictable positions.
In addition to being reliable, despite imprecise location or movability of the protected object, and despite exposure to the weather, a theft detection device needs to be easily operable and to be resistant to damage. Furthermore, such a device, if it includes a magnetically operated switch, needs to be resistant to deception, as by the use of hand-held magnets or by unauthorized removal of the actuating magnet, along with the switch assembly, from the object whose theft is to be detected by the device.
What is needed, therefore, is an improved electrical cable and connecting device for use in conjunction with a security alarm control system which is capable of reliable operation to detect removal of a portable or mobile object from its proper location without being susceptible to weather, and which does not require the object protected to be precisely located or completely stationary.